He is my flake.nix fil :

{
  description = "my flake";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
    nixpkgs_unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
    nix_software_center.url = "github:snowfallorg/nix-software-center";
  };

  outputs = inputs@{ self, nixpkgs, ... }: {

    nixosConfigurations.default = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      specialArgs = { inherit inputs; };
      modules = [
        ./configuration.nix
      ];

    };

  };
}

and here is what I put in configuration.nix :

{ config, pkgs, lib, inputs, ... }:

{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];
  nix.settings.experimental-features = [ "nix-command" "flakes" ];

  environment.systemPackages = with pkgs; [
    nextcloud-client
    inputs.nix_software_center.packages."${pkgs.system}".nix-software-center
    inputs.nixpkgs_unstable.packages."${pkgs.system}".tribler
  ];
}

but if I want to install tribler and I uncomment the line
inputs.nixpkgs_unstable.packages."${pkgs.system}".tribler
then it says

error: attribute 'packages' missing

       at /nix/store/i9ywdccn0czb9bw8gfi9qnk954v33b2w-source/configuration.nix:240:5:

          239|     #unstable.rustdesk-flutter
          240|     inputs.nixpkgs_unstable.packages."${pkgs.system}".tribler
             |     ^
          241|   ];